Calls are mounting for law changes in light of Friday's terrorist attack at a West Auckland supermarket that has several people still fighting for their lives.

Deputy Prime Minister Grant Robertson said today that four people remained in Auckland Hospital.

Two people were in a critical but stable condition, another was stable, and one was critical but improving.

Robertson said on Sunday that prior to the attack occurring, anything and everything authorities could have done was done.

He vowed laws around immigration and terrorism suppression would be changed where needed to prevent future similar situations.

"We have at every turn gone to every part of the law ... left no stone left unturned."

Robertson told Newstalk ZB's Mike Hosking said the reality was, officials could only venture down the deportation avenue which the Government was doing when the attack happened.

"As I said, the Government has been intensely frustrated by this ... we were in that [deportation] process."

Asked who paid for it, Robertson said he couldn't answer that, but given it was around refugee status there would be legal aid involved.

Counter-terrorism specialist Dr John Battersby says Ahamed Aathill Mohamed Samsudeen would have been under the strongest possible surveillance.

But police were not able to sit right on Samsudeen - they did not have the lawful authority to harass someone.

"There isn't a security apparatus in the world that has managed to completely stop terror attacks," he told TVNZ.

There was no perfect security system - even overseas where anti-terrorism measures were much better resourced, they were still not able to stop every attack.

"We do need to wake up and be aware this is going to happen again. We'll possibly stop and prevent and mitigate a number of them but we can't say that it won't happen again."

A 'nutter' needs to act to constitute a crime

Criminal lawyer Steve Cullen told Mike Hosking on Newstalk ZB that the problem with the law in New Zealand if you have a "nutter" planning something there isn't anything to prosecute them with - they have to pounce first.

As for looking at them as an individual, Cullen said he appreciated the protests by mental health groups, the Government just had to make it an offence to prepare for a terror attack: "That's not an offence under criminal law."

In the field of terrorism planning people could strike instantly.

The police were of the view that the planning was a criminal act, one of the problems was the law was passed in 2002, and only used against the mosque terrorist.

Cullen was hoping that the review of the law would be done sooner rather than later.

Queen City Law managing director Marcus Beveridge told Newshub today that most Kiwis would agree the terrorist should have been booted out of the country.

"In terms of the big question you have where most Kiwis would be prejudiced that this guy was about to be at large in society, most people probably think he should have been fish bait particularly because of the comments he made about Kiwis and so on.

"The process took too long, the modification of the law took too long, some of the commentators said the law had enough weight in it just to keep him locked up in any event and all of us would have agreed he should have been booted out at the drop of the hat."

Beveridge said maybe New Zealand needed to adopt a "more Australian type approach to these things" given there are about another 40 people on NZ Security Intelligence Service's watchlist.

Beveridge said there were rules around when a person granted residency could be deported and with refugee law, there was which made it difficult to return someone to their country of origin.

There are now questions around whether evidence the terrorist submitted to defend his refugee application was false.

"Sometimes the wheels of justice with immigration do move very slowly."

Robertson told the AM Show this person arrived in NZ on a student visa and then applied for refugee status which was declined. The person appealed that and was successful.

He said it wasn't until several years later issues arose around his activities. The Government had to wait for the criminal charges to be dealt with before it could deal with his deportation status.

Robertson said he could feel people's frustration about the process. "We went down every single pathway here to see what we could do."

He said the Government was now committed to passing the new law as soon as it could. The PM's first briefing on this was in 2018 and she recalls asking about deportation at that time, he said. The Government would now look at whether it could keep someone detained while looking at a deportation order.

Responding to the terrorist's mother's claims he was radicalised by his neighbours, Robertson said the only evidence he had was that it was a "lone-wolf attack" and that he had accessed material online. "The police have been clear it was a person acting alone..."

Robertson said the Isis bride now in New Zealand with her young children had been looked at clearly and no stone had been left unturned.

"What I can say with absolute certainty is Police and all the relevant agencies are managing this situation carefully and to make sure everybody in the situation is safe."

He would not say whether she was living in the community or under close surveillance.

On Friday afternoon 32-year-old Samsudeen was shot dead by police from the Special Tactics Group after the terrifying incident in which five people were stabbed and two others injured.

Samsudeen - described by Prime Minister Jacinda Ardern soon after the attack as a terrorist - was an identified threat, a dangerous high risk to the public, on a terror watch-list, and was under 24/7 surveillance.

Robertson said as part of law changes they were looking into the Immigration Act.

Samsudeen had arrived in New Zealand in October 2011 from Sri Lanka and was granted refugee status two years later.

Ardern said it turned out he had used fraudulent documentation. Officials tried to revoke his refugee status in 2018, but he appealed and a final decision was yet to be made on whether he could be deported.

"The Government at every turn sought a remedy for this and at every turn we found we weren't able to [deport him]," Robertson said.

National leader Judith Collins told TVNZ's Q and A her party would back law changes and wanted to work with the Government.

She wanted the Government to be able to strip citizenship or residency from those who have moved to New Zealand but then committed a violent act.

The man had allegedly been planning a knife attack earlier this year, but he couldn't be charged with planning a terrorist attack because it wasn't an existing offence.

It is, however, an offence in the Counter-Terrorism Legislation Bill, currently before a select committee, which the National Party supported at first reading.

Act Party leader David Seymour said they wanted to see changes to the Immigration Act that would allow people to be detained and ultimately deported if they posed a threat to New Zealand.

They also supported making preparing to commit terrorism a crime.

Meanwhile, law experts have been urging caution around rushing through legislation that could have unintended consequences.

Legal expert Andrew Geddis told RNZ the proposed legislation meant "even just thinking about doing it will be an offence for which you could go to jail for up to seven years".

It was also not clear if the man would have been sentenced to jail if the law had already been changed.

He had already been convicted of possessing material that promoted terrorism, which could be punished by up to 10 years' jail, but the judge chose a supervision sentence, meaning he was allowed to be in the community with conditions, including undergoing a psychological assessment.

There had also been questions around whether Samsudeen could have been detained and committed under the Mental Health Act, given his mental health and that he was being monitored by authorities after serving a prison sentence.

Robertson said it was not possible to detain Samsudeen as he did not meet the criteria.

Lawyer Alden Ho, of Crimson Legal, told the Herald it was not possible for him to be detained as he had refused a psychological assessment.

"A further difficulty is how the Act construes the definition of 'mental disorder' which excludes a person's political, religious or cultural beliefs."

The Mental Health Foundation also said in a statement terrorist ideologies were not symptoms of any mental illness.

The Mental Health Act was not to be used as "a stopgap to plug holes in criminal law".

"It should never – ever – be used punitively. Mental health support is not a punishment. It's a human right."